In certain applications utilizing a switch-mode power supply, a secondary, lower voltage output is required to be maintained and regulated for a predetermined time period, after a loss of input power while the main output of the power supply supports a minimal load. Since the secondary output may be post regulated, the secondary output will shutdown as soon as the regulator input voltage falls below the dropout voltage of the regulator. The addition of input bulk capacitance to the power supply in order to maintain the secondary output may not be practical since the energy would be transferred to all the outputs, resulting in increased holdover for all the outputs, and not achieving the required holdover for a desired specific output. Other holdover techniques, such as, for example, electronic shutdown of the remaining outputs, may not be practical or cost effective. Adding additional capacitance to the secondary output is also not practical since this addition would require the storage of an excessively large amount of energy to achieve the required holdover.
A need has thus arisen for a holdover circuit for diverting energy to a secondary output that is post regulated, after the loss of input power.